Microporous zeolite nanocrystals

Figure 2.2 Classification of different types of porous materials, (a) A purely microporous zeolite is considered as a non-hierarchical system according to the single level of porosity, (b) Fragmentation of the zeolite into nanocrystals engenders a network of mesopores constituting the intercrystalline space, leading to an interconnected hierarchical system. Intraconnected...

L Huang, Z. Wang, H. Wang, J. Sun, Q. Li, D. Zhao, and Y. Yan, Hierarchical Porous Structures by Using Zeolite Nanocrystals as Building Blocks. Microporous Mesoporous Mater, 2001, 48, 73-78. 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

A new approach for the synthesis of mesostructured zeolitic materials (namely UL-TS-1 and UL-ZSM-5) is reported. The materials were obtained in the solid state by heating TPAOH-impregnated mesoporous materials for several days. Various techniques including XRD, N2 adsorption, UV-visible, FTIR, TEM and Si MAS NMR were used to monitor the physicochemical properties of these materials as a function of crystallization time. The increase in the percentage of crystallinity is correlated with the corresponding variations in micropore and mesopore volumes, BET and BJH surface areas. The results indicate that the mesopore walls consist of zeolite nanocrystals. Depending on crystallization time, a range of materials from totally amorphous up to 80% crystalline is observed, while some of mesopores are preserved. 

Huang L, Wang Z, Wang H, Sun J, Li Q, Zhao D, Yan Y. Hierarchical porous structures by using zeolite nanocrystals as building blocks. Micropor Mesopor Mater 2001 48(1-3) 73-78. 

In addition, significant advances have been made in both basic and applied research which allow a smart and efficient solution to most of these problems. As an example, let us quote the development of the synthesis of novel catalytic materials with tailor-made and more suitable characteristics (stable nanocrystals, controlled hydrophobicity, better thermal and/or mechanical stability, etc.), the understanding of the complex phenomena involved in the catalytic transformation of polar molecules within zeolite micropores or the demonstration that fixed bed reactors, which have many advantages over conventional batch reactors, can be easily used, even for liquid-phase reactions and even for laboratory scale experiments. 

B. A. Holmberg, H. Wang, J.M. Norbeck, and Y. Yan, Controlling Size and Yield of Zeolite Y Nanocrystals Using Tetramethylammonium Bromide. Microporous Mesoporous Mater, 2003, 59, 13-28. 

Another approach to introduce mesoporous channels to give better access of reactant molecules to the microporous regions is to assemble zeolite nanoparticles around micellar templates, in a modification of the standard route to mesoporous silicas. Reported examples include structures that possess walls made out of nanocrystals of zeolites such as Beta or ZSM-5. These composite solids possess enhanced hydrothermal stabilities and acidities compared to mesoporous solids with fully amorphous walls. The improved properties are attributed to the presence of the zeolite fragments, because zeolites are known to have higher acidity and hydrothermal stability than amorphous silica/... 

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